Various methods for production of acetic acid from ethylene in a single-stage reaction have been proposed, for the numerous advantages in terms of industrial production steps and economy. For example, there have been disclosed a liquid-phase single-stage oxidation process using oxidation-reduction catalysts with metal ion pairs such as palladium-cobalt and palladium-iron (French Patent No. 1448361), processes using catalysts composed of palladium-phosphoric acid or sulfur-containing modifiers (Japanese Unexamined Patent Publication No. 47-013221, Japanese Unexamined Patent Publication No. 51-029425), and a gas-phase single-stage oxidation process using a catalyst composed of a three-component oxygen compound (Japanese Examined Patent Publication No. 46-006763). As a method for producing acetic acid using a catalyst comprising a palladium compound and a heteropoly acid, there has been proposed a gas-phase single-stage oxidation process using palladium phosphovanadomolybdate (Japanese Unexamined Patent Publication No. 54-57488).
Recently, as catalysts for obtaining acetic acid from ethylene and oxygen there have been proposed catalysts having palladium metal and an element of Group 14, 15 or 16 of the Periodic Table supported on a carrier (Japanese Unexamined Patent Publication No. 11-347412). These supported catalysts are prepared by the following steps.
First step: A step of loading a palladium-containing compound onto the carrier.
Second step: A step of alkali treatment.
Third step: A step of reduction treatment of the palladium-containing compound to produce palladium metal.
Fourth step: A step of loading an element of Group 14, 15 or 16 of the Periodic Table.
“Eggshell-type” palladium catalysts are useful for such supported catalysts. Eggshell-type catalysts are catalysts having the palladium in the carrier positioned on the surface of the carrier. Because reaction substrates do not readily diffuse into the internal regions of catalyst carriers, the metal components supported in the carrier have low probability of contacting the reaction substrates, and therefore their contribution to the reaction is low. With eggshell types, the metal component is abundantly present on the carrier surface, so that the efficiency for the reaction is higher than with normal types, even with the same amount of metal component. One known production process for obtaining eggshell-type palladium catalysts comprises a step of alkali treatment with sodium metasilicate or the like (Japanese Unexamined Patent Publication No. 7-89896). Also, Japanese Unexamined Patent Publication No. 2000-308830 discloses a process for production of an eggshell-type palladium supported catalyst comprising a step of treatment with an alkaline earth metal salt such as barium hydroxide.
The production process for a catalyst for production of acetic acid disclosed in Japanese Unexamined Patent Publication No. 11-347412 has a step in which a third component is loaded after an alkali treatment step to asymmetrically load the metal component such as palladium on the carrier surface (eggshell formation). The catalyst obtained by this process has high catalytic activity, but is problematic in that the catalyst preparation process is long and catalyst deterioration occurs during the reaction. The present inventors have therefore pursued development of a simple preparation process for catalysts that can prevent deterioration while maintaining high activity.
Another factor is that carbon dioxide is generated as a by-product in acetic acid production processes involving reaction between ethylene and oxygen. For example, according to Japanese Unexamined Patent Publication No. 7-89896, the carbon dioxide selectivity is about 5%. Generation of carbon dioxide means a lower final yield of acetic acid. In recent years, it has become a major goal to minimize production of carbon dioxide from the viewpoint of preventing global warming and reducing the environmental load. From an industrial standpoint, expensive equipment investment and equipment operation and maintenance costs are necessary for treatment of carbon dioxide by-product. The present inventors therefore investigated methods of further reducing carbon dioxide by-product in the production of acetic acid.